A constructed language (shortened to conlang)[a] is a language whose phonology, grammar, orthography, and vocabulary, instead of having developed naturally, are consciously devised for some purpose, which may include being devised for a work of fiction. A constructed language may also be referred to as an artificial, planned or invented language, or (in some cases) a fictional language. Planned languages (or engineered languages/engelangs) are languages that have been purposefully designed; they are the result of deliberate, controlling intervention and are thus of a form of language planning.
There are many possible reasons to create a constructed language, such as to ease human communication (see international auxiliary language and code); to give fiction or an associated constructed setting an added layer of realism; for experimentation in the fields of linguistics, cognitive science, and machine learning; for artistic creation; for fantasy role-playing games; and for language games. Some people may also make constructed languages as a hobby.
In terms of purpose, most constructed languages can broadly be divided into:
A priori language
An a priori language is any constructed language with some features which are not based on existing languages. Instead these features are invented or elaborated to work differently or to allude to different purposes. Some a priori languages are designed to be international auxiliary languages that remove what could be considered an unfair learning advantage for native speakers of a source language that would otherwise exist for a posteriori languages. Others, known as philosophical or taxonomic languages, try to categorize their vocabulary, either to express an underlying philosophy or to make it easier to recognize new vocabulary. Finally, many artistic languages, created for either personal use or for use in a fictional medium, employ consciously constructed grammars and vocabularies, and are best understood as a priori.
A posteriori language
An a posteriori language (from Latin a posteriori, from the latter), according to French linguist Louis Couturat, is any constructed language whose elements are borrowed from or based on existing languages. The term can also be extended to controlled versions of natural languages, and is most commonly used to refer to vocabulary despite other features. Likewise, zonal auxiliary languages (auxiliary languages for speakers of a particular language family) are a posteriori by definition.
While most auxiliary languages are a posteriori due to their intended function as a medium of communication, many artistic languages are fully a posteriori in design—many for the purposes of alternate history. In distinguishing whether the language is a priori or a posteriori, the prevalence and distribution of respectable traits is often the key.